1. Field of the Invention
The present invention relates to a method for manufacturing a display device.
2. Description of the Related Art
In recent years, thin film transistors each formed using a semiconductor thin film with a thickness of several nm to several hundred nm over a substrate having an insulating surface, such as a glass substrate, have attracted attention. Thin film transistors have been widely used for ICs (integrated circuits) and electronic devices typified by electrooptic devices. Thin film transistors have been rapidly developed particularly as switching elements for display devices typified by liquid crystal display devices, EL (electroluminescent) display devices, and the like. In an active matrix liquid crystal display device, a voltage is applied between a selected pixel electrode connected to a switching element and a counter electrode corresponding to the selected pixel electrode, whereby a liquid crystal layer disposed between the selected pixel electrode and the counter electrode is subjected to optical modulation, which is perceived by a viewer as a display pattern. Here, an active matrix liquid crystal display device refers to a liquid crystal display device adopting a method in which pixel electrodes arranged in matrix are driven by switching elements to display a display pattern on a screen.
The range of uses of such an active matrix liquid crystal display devices described above has been increased and thus, demands for a larger size of a screen, higher definition, and a higher aperture ratio have been increased. Further, higher reliability of an active matrix liquid crystal display device is required and high productivity and reduction in manufacturing cost are required for a method for manufacturing the active matrix liquid crystal display device. Simplification of a process is one way for increasing productivity and reducing manufacturing cost.
In an active matrix liquid crystal display device, thin film transistors are mainly used as switching elements. In manufacturing thin film transistors, it is important that the number of photomasks to be used for photolithography be reduced in order to simplify a process. For example, when one more photomask is required, the following steps are further required: application of a resist, prebaking, light exposure, development, postbaking, and the like, and steps before and after the above steps, such as formation of a film, etching, removal of a resist, cleaning, and drying. Thus, even when only one more photomask is used in a manufacturing process, the number of steps is significantly increased. In order to reduce the number of photomasks used in a manufacturing process, many techniques have been developed.
Thin film transistors are broadly classified into top gate thin film transistors in each of which a channel formation region is provided below a gate electrode and bottom gate thin film transistors in each of which a channel formation region is provided above a gate electrode. It is known that the number of photomasks used in a process for manufacturing a bottom gate thin film transistor is smaller than that in a process for manufacturing a top gate thin film transistor. A bottom gate thin film transistor is generally manufactured using three photomasks.
Further, as conventional techniques for reducing the number of photomasks, complicated techniques such as backside light exposure, resist reflow, and a lift off method are often used and a particular apparatus is required in many cases. It has been concerned that using such complicated techniques may cause various problems, which leads to reduction in yield and the like. Further, the number of photomasks has been reduced at the cost of electric characteristics of a thin film transistor in many cases.
As a typical means for reducing the number of photomasks in a process for manufacturing thin film transistors, a technique using a multi-tone mask (referred to as a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing manufacturing steps by using a multi-tone mask, for example, Patent Document 1 (Japanese Published Patent Application No. 2003-179069 is given. By using a multi-tone mask, a bottom gate thin film transistor can be manufactured using two photomasks.